In recent years, enhancement of fuel economy of the automobile has become an important issue from the viewpoint of global environmental conservation. Consequently, there is an active movement afoot to reduce the thickness through increases in strength of car body materials, so as to reduce the weight of a car body itself. However, an increase in strength of a steel sheet causes reduction in elongation, that is, reduction in formability. Therefore, development of materials having high strength and high workability in combination has been desired under the present circumstances.
Furthermore, in forming of the high strength steel sheet into a complicated shape, e.g., an automobile component, occurrences of cracking and necking in a punch stretch portion or a stretch flange portion cause large issues. Therefore, a high strength steel sheet which can overcome the issues on occurrences of cracking and necking and which has high elongation and high stretch flangeability in combination has also been required.
Regarding an improvement of formability of the high strength steel sheet, heretofore, various multi phase high strength galvanized steel sheets, e.g., a ferrite-martensite dual-phase steel (Dual-phase Steel) and a TRIP steel taking the advantage of the transformation induced plasticity (Transformation Induced Plasticity) of retained austenite phase, have been developed.
For example, PTLs 1 and 2 have proposed steel sheets with excellent elongation by specifying the chemical components and specifying the volume fractions of retained austenite phase and martensitic phase and methods for manufacturing the same. Moreover, PTL 3 has proposed a steel sheet with excellent elongation by specifying the chemical components and, furthermore, specifying a special method for manufacturing the same. In addition, PTL 4 has proposed a steel sheet with excellent elongation by specifying the chemical components and specifying the volume fractions of ferrite phase, bainite phase, and retained austenite phase.